The production of electronic devices such as lead frames, lead frame packages, connectors and printed circuit boards requires silver spot plating followed by solder-plating to improve the corrosion resistance and solderability thereof. Sn--Pb solder has been usually employed in such solder-plating process; however, the toxicity of the lead component poses a serious environmental hazard.
Accordingly, there have been many attempts to ameliorate the lead problem but a cost-effective solution has not yet been attained. For example, a method to plate palladium or gold on a Cu--Ni primer plating layer has been suggested, but this method has the problem of a high production cost.
The present inventors have endeavored to develop a new plating composition that can replace the environmentally hazardous Sn--Pb solder-plating composition, and have unexpectedly discovered that several novel tin-based alloy plating compositions are capable of providing solder plating layers having excellent corrosion resistance and solderability.
Hitherto, many kinds of tin-based alloy plating compositions have been reported. For example, U.S. Pat. No. 4,828,657 (Fukuoka et al., filed on May 9, 1989) describes tin-cobalt, tin-nickel and tin-lead binary alloy plating compositions, and U.S. Pat. No. 4,347,107 (Teichmann et al., filed on Aug. 31, 1982) mentions the deposition of tin as well as copper or rhodium alloys thereof on various substrates. However, these compositions are applied to ensure the deposition of bright metallic tin for a decorative purpose and are not suitable for solder plating electronic devices or for replacing the Sn--Pb solder-plating.
In addition, a tin-bismuth alloy plating composition containing a chelate salt such as an acid bismuth sulfate gluconate is described in U.S. Pat. No. 4,331,518 (Wilson, filed May 25, 1982); and a tin-nickel alloy plating composition comprising a fluoride is disclosed in U.S. Pat. No. 4,049,508 (Morrissey, filed Sep. 20, 1977). These compositions, however, give plating layers having poor solderability and emit harmful materials to the environment.